Light cover

ABSTRACT

A light cover has a transparent element manufactured from polycarbonate and an infrared filter angled so as to reflect infrared radiation emitted from a light source way from both the transparent element and the light source.

This application is the US national phase of international applicationPCT/GB03/00258 filed 24 Jan. 2003, which designated the US and claimspriority to GB Application No. 0202299.4, filed 01 Feb. 2002. The entirecontents of these applications are incorporated herein by reference.

The present invention relates to a light cover arranged to be placedover a light source, and in particular a light cover in combination witha reflective element.

It is common practice within the transport industry to place transparentcovers over exterior light sources on moving vehicles or aircraft. Aswell as protecting the light sources from damage, such as from being hitby small stones or debris that may strike the moving vehicle from timeto time, the covers allow good aerodynamic profiles to be achieved.

Within the aircraft industry, all exterior lights, such as navigationand landing lights, are required by safety legislation to have suchcovers. In particular, for aircraft landing lights the safetyregulations stipulate that in the event that the light cover for thelanding light is broken or cracked then the aircraft must not fly untilthe cover has been replaced.

This can prove problematic to both the aircraft operator and theaircraft manufacturer. It is clearly detrimental to the aircraftoperator because of the direct cost of replacing landing light covers ona frequent basis, together with the indirect costs incurred due to theaircraft being temporarily out of service. Furthermore, in order toprovide protection against the possibility of difficulties in sourcingor manufacturing sufficient quantities of light covers to satisfy theaircraft operators demand, for example because of industrial action, theaircraft manufacturer may need to hold large numbers of covers in stock,to the detriment of their cash flow.

At present this potential problem of second sourcing is exacerbatedbecause the light covers are made of glass. Although glass has excellentoptical qualities, the complex shapes often required for the lightcovers, due to the need to present a smooth aerodynamic surface, can beexpensive to manufacture. Furthermore, glass is known to be a relativelybrittle and inflexible material that can be broken by the stresses andstrains that are inevitably transmitted to the light cover due to theflexing of the various aircraft surfaces even though strain relief isprovided for the covers. Although transparent plastic materials areknown that could be manufactured to the required shape and have thenecessary strength and flexibility to withstand such stresses andstrains without cracking or breaking, they have hitherto remained unusedbecause they melt under the heat of the landing light bulb, the bulbsbeing typically 600 watts. It will, of course, be appreciated that thespace available within an aircraft is restricted and hence the bulbs arerelatively close to the cover. For example, a polycarbonate light coverfor a 600 watt landing light bulb would melt in approximately 10minutes. Although this would be permittable for emergency use, it isclearly not a viable proposition as a permanent solution.

According to a first aspect of the present invention there is provided alight cover arranged to be placed over a light source, the light sourcefurther emitting infrared radiation when in use, wherein the light coverincludes a reflective element arranged to reflect a portion of theemitted infrared radiation away from the light cover.

Preferably the reflective element comprises an infrared reject filter.Preferably the reflective element is partially mirrored.

Preferably the light cover is manufactured from a synthetic material,such as a polycarbonate plastic. Preferably the light cover is coatedwith a scratch resistant layer such as Perspex (registered trade mark).

Preferably, said reflective element is further arranged to reflect saidinfrared radiation away from said light source.

Preferably the light cover is an aircraft exterior light cover.

According to a second aspect of the present invention there is provideda light assembly comprising a light source mounted within a housing, thelight source emitting infrared radiation when in use, a light coverplaced over said light source and a reflective element located betweensaid light source and said light cover and arranged to reflect a portionof the emitted infrared radiation away from the light cover.

It is therefore possible to provide a light cover that is both strongand flexible enough so as to not be prone to cracking or breaking, andis capable of withstanding the heat generated from the light bulb.

An embodiment of the present invention is described, by way of exampleonly, with reference to the accompanying figures, of which:

FIG. 1 is a perspective view of a light cover according to an embodimentof the present invention; and

FIG. 2 is a schematic representation of a light assembly including thelight cover shown in FIG. 1.

FIG. 1 shows a perspective view of a light cover according to anembodiment of the present invention. The cover comprises a transparentelement 2 that is substantially the size and shape of the required lightcover. Transparent element 2 has a shaped periphery (not visible inFIG. 1) that is fastened between an outer mounting ring 4 and an innerfastening ring 6. The inner fastening ring 6 is secured to the outermounting ring 4 using a number of screws or bolts 8, thus securelyfastening the transparent element 2 between it and the outer mountingring 4. The shaped periphery of the transparent element 2 is such that asmooth aerodynamic profile is presented by the exterior surface of thelight cover.

Mounted on the inner fastening ring 6 are two filter support brackets10. Attached to the filter support brackets 10 so as to span across aportion of the transparent element 2, is an infrared reject filter 12.Such filters are commercially available. The infrared filter 12, due tothe configuration of the filter mounting brackets 10, is inclined to thesurface of the transparent element 2.

FIG. 2 schematically illustrates the positioning and operation of alight assembly 14 incorporating a light cover as shown in FIG. 1. Thelight assembly 14 comprises a light source 16 mounted within a housing18 and covered by a light cover, only the transparent element 2 of whichis shown in FIG. 2 for clarity. The light assembly 14 is typicallymounted within the wing and/or fuselage 20 of an aircraft.

The transparent element 2 of the light cover is preferably manufacturedfrom polycarbonate, although other suitable synthetic materials may beused. Polycarbonate is a strong, transparent thermosetting plastic thatcan be moulded and/or machined to the complex shape required for thelight cover, yet is flexible enough to withstand the stresses andstrains transferred to the light cover from the wing and/or fuselage. Aspolycarbonate is extremely difficult to polish, and the exterior surfaceof the light cover is prone to being scratched due to dust abrasion, itis preferred that the exterior surface of the transparent element 2 iscoated in Perspex (registered trade mark) or the like. Perspex is moreeasily polished, allowing the majority of the scratches to be polishedout, thus extending the life of the light cover.

Referring back to FIG. 2, the arrow 22 represents a light ray from thelight source 16. The light ray passes through the infrared filter 12 andthe transparent element 2 to illuminate an area in front of the lightassembly 14. The light source 16, which may typically be a 600 wattbulb, also emits a substantial amount of heat as infrared radiation. Theinfrared filter 12, which may be coated so as to render the filter 12partially mirrored, reflects a substantial proportion of the infraredradiation, indicated by the chain arrow 24, towards the rear of thehousing 18. Without the presence of the infrared filter 12, the heatemitted from the light source 16 would be sufficient to melt thepolycarbonate transparent element 2 in approximately 10 minutes.

The angle of the infrared filter 12, indicated by angle A, is carefullyselected to direct the reflected infrared radiation towards anon-critical part of the lighting housing 18 so as to avoid unwantedheating of a heat sensitive part of the assembly. It is important thatthe infrared radiation is not directed substantially back towards thelight source 16, as this would cause the light source itself to overheatand fail. It will be appreciated that the infrared filter 12 mayalternatively be secured to the housing 18 by suitable means whilststill reflecting infrared radiation away from the transparent element 2.

The present invention therefore provides a light cover incorporating orassociated with a heat reflector that, with careful selection of theangle of the heat reflector, directs heat away from both the light coveritself and the light source. This allows plastics which have hithertobeen precluded from use due to thermal problems to be used.

1. A light cover arranged to be placed over a light source emittinginfrared radiation, wherein said light cover comprises an aircraftexterior light cover, wherein the light cover includes a reflectiveelement angled with respect to said light cover for reflecting a portionof said emitted infrared radiation away from said light cover andsubstantially away from said light source.
 2. A light cover according toclaim 1, wherein said reflective element comprises an infrared rejectfilter.
 3. A light cover according to claim 1, wherein said reflectiveelement is partially mirrored.
 4. A light cover according to claim 1wherein said light cover is manufactured from a synthetic material.
 5. Alight cover according to claim 4, wherein said synthetic material ispolycarbonate.
 6. A light cover according to claim 4, wherein said lightcover is coated with a scratch resistant layer.
 7. A light coveraccording to claim 1, wherein said reflective element is furtherarranged to reflect said infrared radiation away from said light source.